I. Field of the Invention
The present invention relates to a steering gear for a vehicle and, more particularly, to a power steering gear or a steering gear having a steering correction function for causing a vehicle to perform a steering correction when disturbance acts on the vehicle, wherein a fail-safe mechanism is provided to allow manual steering when a power assist device or a control device fails.
II. Description of the Prior Art
A typical conventional fail-safe mechanism used in a steering gear for a vehicle is shown in FIG. 2. A failure in a steering drive unit 230 or a steering drive control device 240 is electrically detected by an abnormality detection sensor 240a. An electrical signal is output from the steering drive control device 240 to actuate a connecting actuator 270 and a function halt actuator 290 to engage an input device 210 with a steering device 220, and to halt the function of the steering drive device 240. Reference numeral 260 denotes a steering control device. A typical example of such a steering gear for a vehicle is shown in FIG. 3 (Japanese Laid-Open Utility Model Publication No. 61-35082).
The arrangement and the principle of operation of the above steering gear for a vehicle will be briefly described. An input shaft 323 is connected to a steering wheel 322. A steering angle detection sensor 326 is connected to the input shaft 323 to detect a steering angle. The input shaft 323 is connected to an output shaft 324 through a clutch means 325. An actual steering angle detection sensor 327 is arranged on the output shaft 324. The output shaft 324 is connected to a pinion 332 through joints 330 and 331. The clutch means 325 is operated by an actuator 329. The pinion 332 is meshed with a rack 333. A power cylinder 337 is connected integrally with the rack 333. The rack 333 is connected to a wheel 336 through a linkage 334 and a knuckle arm 335. Oil from a hydraulic source 338 is selectively supplied to oil chambers 337a and 337b by an electromagnetic spool valve 344. A pressure-sensitive switch 343 is arranged in a line between the hydraulic source 338 and the electromagnetic spool type valve 344. A clutch valve 345 is arranged in lines between the electromagnetic spool type valve 344 and the oil chambers 337a and 337b in the power cylinder 337 to connect the two lines. Signals from a vehicle velocity sensor 346, the steering angle sensor 326, the steered angle sensor 327, and the pressure-sensitive switch 343 are input to a control device 328. Control signals are output from the control device 328 to the electromagnetic spool type valve 344, the actuator 329 in the clutch means 325 and the clutch valve 345.
In the normal operation, the clutch means 325 and the clutch valve 345 are kept in the OFF state. The electromagnetic spool type valve 344 is controlled in accordance with signals from the steering angle detection sensor 326 and the steered angle detection sensor 327, and steering is performed in accordance with a rotational angle of the steering wheel 322. However, when the control device 328 receives through an OR gate a hydraulic failure signal from the pressure-sensitive switch 343, a controller abnormality signal, or a vehicle velocity signal representing that a vehicle velocity is higher than predetermined vehicle velocity, a control signal is output from the control device 328 to actuate the clutch means 325 and the clutch valve 345. Then, the input shaft 323 is connected to the output shaft 324, and at the same time, the oil chambers 337a and 337b in the power cylinder 337 communicate with each other. In this state, the steering gear allows manual steering requiring a small steering force. Therefore, steering in an abnormal state is guaranteed and steering during high-speed traveling can be assured, thereby assuring safety.
In the conventional steering gear for a vehicle described above, however, the clutch means for connecting the input and output shafts and the means for halting the power assist function as the steering drive unit are controlled by the sensors and the control device. Therefore, when the sensors fail or the control device malfunctions, manual steering cannot be performed and safe travel cannot be assured. Assume that the pressure-sensitive switch 343 fails to detect a change in pressure under the condition wherein an abnormality occurs in the hydraulic source 338 and a predetermined pressure cannot be obtained. Since the abnormality of the hydraulic source 338 is not detected as an abnormality by the control device 328, the clutch means 325 is not set in an engagement state and steering becomes impossible. Therefore, safe travel is not assured. When the control device 328 malfunctions to halt its function including the fail-safe function, normal steering cannot be performed. In addition, since no abnormality detection signals are output to the actuator 329 in the clutch means 325 and the clutch valve 345, manual steering cannot be performed either.